Each carbon molecule can bond with as many other atom(s) or molecule(s).
one
two
six
four

Option D, Four.

Explanation:

A carbon atom has four  electron in its outermost shell. Thus in order to complete its octate (which has 8 electrons) a carbon needs to form four bonds. Thus a carbon can bond with either 4 other atoms or molecules.

For example -

CH4

Carbon has 4 electron in its outermost shell. Four more electrons are donated by hydrogen atom with each hydrogen atom fulfilling one valency of carbon. Thus now carbon has 8  electron in its outermost shell.

Thus, option D is correct.